My Stag AR lower has multiple uppers to answer that question. I can swap on the .22LR upper for plinking, or swap on another caliber for hunting up to .50 Beowulf. The fit and feel of the rifle is almost the same until the shot breaks and you just have to have the appropriate sight set up on each upper. The alternative is 4 or 5 separate rifles. Need a bigger safe for those.

So that's one line of reasoning. Everyone looks at it differently. No one right answer.